Sending hugs. First off I know how you feel about rudeness, I've had two bad experiences in quilt classes and I don't like being put down either. There are no excuses.
About mitered corners, I could never get the knack of doing them properly until I saw a Missouri Star tutorial about binding your quilt. Jenny Doan makes it so simple and it works. You can just Google Missouri Star quilt Company and there will be a link for tutorials. Good luck with this and don't give up on your good work.